This invention relates to an electric power steering apparatus for an automobile which employs an electric motor to decrease the steering torque which must be exerted by a driver in order to steer the automobile. More particularly, it relates to an electric power steering apparatus which can guarantee safe operation.
Electric power steering apparatuses for automobiles are well known. In these apparatuses, an electric motor is connected to a steering shaft of an automobile by a clutch. The application of torque to the steering wheel by the driver of the automobile is detected by a torque sensor, which produces a corresponding output signal. Based on the output signal, a CPU controls the electric motor through a motor driver so as to rotate the steering shaft in a direction which will decrease the torque which need be applied by the driver in order to steer the automobile.
Conventional electric power steering apparatuses have a number of problems with respect to safety. For example, when there is a ground fault in a portion of the motor driver for the electric motor, it is possible for the electric motor to be driven, even though the driver is not applying any torque to the steering wheel. This produces the extremely dangerous situation of the automobile being steered in a manner contrary to the will of the driver.
Sometimes, due to a problem with the power steering apparatus, a directional command signal which is input to the motor driver does not agree with the direction of torque which is applied to the steering wheel by the driver of the automobile. If the motor driver is operating normally, no power will be supplied to the motor in this situation, but if there is a ground fault in the motor driver, current may end up being supplied to the motor, and the wheels of the vehicle may be steered in the direction opposite to the direction desired by the driver, which is of course highly dangerous.
Another problem is that the electric motor can exert a torque on the steering shaft even in situations in which power steering is unsafe, such as when the vehicle is traveling at a high speed and the driver applies a large torque to the steering wheel.